1. Field of the Invention
The present invention relates to a carbon material used as an activating material in the negative electrode of a lithium-ion secondary battery or in a polarizable electrode in an electrical double-layer capacitor. More particularly, the invention relates to a method of efficiently removing electrochemically active oxy-hydrogens (hydrogen atoms bonded to oxygen atoms in the natural state, e.g., hydrogen atoms existing as hetero element-containing functional groups, such as COOH, CHO, and phenolic OH) at a relatively low temperature. Where an electrode is fabricated from such a carbon material, the aforementioned electrochemically active oxy-hydrogens are one of the causes of inability to achieve a high-voltage operation, and are present on the surface of the carbon material or within micropores.
2. Description of Related Art
In an electrical double-layer capacitor, a pair of polarizable electrodes is placed opposite to each other within an electrolyte solution via a separator to form positive and negative electrodes. The capacitor operates on the principle that electric charge is stored in an electrical double-layer capacitor formed at the interface between each polarizable electrode and the electrolyte solution. Therefore, it is considered that the capacity of an electrical double-layer capacitor is roughly proportional to the surface area of the polarizable electrode. Hence, porous carbon and activated carbon having specific surface areas of 1,000 to 2,000 m2/g, as measured using the BET method, have been often used as activating materials for polarizable electrodes (for example, Hiratsuka Kazuya et al., DENKI KAGAKU, Vol. 59, No. 7, pp. 607–613 (1991)).
On the other hand, a nonporous carbon having only a specific surface area of less than 100 m2/g has been proposed (M. Takeuchi et al., DENKI KAGAKU, Vol. 66, No. 12, pp. 1311–1317 (1998)). In all of these carbon materials including porous carbon, activated carbon, and nonporous carbon, activation leaves a large number of functional groups on carbon surfaces. Where a polarizable electrode is made of such a carbon material, if an organic solvent-based electrolyte solution is used, and if a voltage of higher than 2.5 V is applied between the opposite electrodes, functional groups (especially, hetero element-containing functional groups) left on the carbon electrode surface react with the electrolyte solution, producing gas or forming an electrically nonconducting film. This, in turn, increases the internal resistance, causing a malfunction, reducing the service life, or producing other problems. High-temperature processing under vacuum has been considered as means for removing such residual functional groups with unsatisfactory results.
This high-temperature processing is based on the process in which H2O, COOH, CHO, C=0, and so on are released as CO2, H2O, and CO within an inert gas stream as the temperature rises, as described in the Carbon Black Handbook (in Japanese), New Edition, p. 11, FIG. 9, “Composition of Volatile Components of Carbon Black” that is a reference regarding carbon. Release in the form of H2 begins when the temperature exceeds about 800° C. Accordingly, where a heat treatment is made at a temperature between 200° C. and 800° C., some contribution is given to removal of residual functional groups. As the temperature is increased, more kinds of functional groups are released. That is, most hetero element-containing functional groups are once removed at 800° C. However, free radicals are produced on the carbon surface at the same time. The produced free radicals are highly reactive and react with H2O and O2 within the air quickly or slowly when carbon is taken out into the air. As a result, electrochemically active oxy-hydrogens (e.g., COOH, CHO, and OH) again form. This can be easily confirmed by observing carbon materials, to which water vapor is artificially added, by pulsed NMR spectroscopy (described later).
In view of the foregoing problem, we have proposed a method (Japanese Patent Application No. 2000-201849, filed Jul. 4, 2000) that is an improvement over the aforementioned high-temperature processing in anticipation of terminating produced radical groups with hydrogen. In particular, the produced radical groups are heat-treated within a hydrogen stream or, industrially, within mixture gas 3H2+N2 obtained by decomposing NH3 gas using Fe2O3 catalyst.
It is reported that the high-temperature processing within this hydrogen stream produces greater effects with increasing the temperature.
However, where the material is used as a polarizable electrode in an electrical double-layer capacitor or the like, nonporous carbon has few micropores in the initial phase, unlike activated carbon. The nonporous carbon forms an electrical double layer by electrochemical intercalation. High capacitance is subsequently maintained by the hysteresis effect. Where the nonporous carbon is post-treated, the interplanar spacing d002 of graphite-like layers grown within the carbon structure varies as shown in FIG. 1 by the temperature of the post-heat treatment. In FIG. 1, the value of the interplanar spacing d002 obtained by X-ray diffraction (XRD) is plotted on the vertical axis, while the heat treatment temperature is plotted on the horizontal axis. The values of interplanar spacing d002 of calcined carbon and KOH-activated carbon, respectively, are indicated around the left end of the horizontal axis. It can be seen from this graph that the interplanar spacing shows a maximum value after KOH activation and that the spacing is reduced by subsequent thermal processing. The general tendency is that the interplanar spacing d002 decreases with increasing the temperature. What are plotted indicate the following carbon materials: Indicated by ▾ is nonporous carbon D prepared using petroleum pitch made infusible as raw material carbon. Indicated by ∇ is nonporous carbon A prepared using petroleum-based needle coke as raw material carbon. Indicated by ▪ is nonporous carbon C prepared using petroleum-based needle coke as raw material carbon. Indicated by ●, ▴ and □ are nonporous carbons B prepared using petroleum-based needle cokes as raw material carbons under different pretreatment and activation conditions. The interplanar spacing d002 tends to decrease with increasing the temperature. As the interplanar spacing d002 decreases, the intercalation start voltage increases. This reduces the hysteresis effect after initial charging.
The capacitance tends to decrease when the operation is performed at a low voltage (FIG. 2). FIG. 2 shows the results of measurements performed by measuring charging and discharging characteristics while increasing the applied voltage in increments of 0.5 V or 0.25 V until the voltage reached 0.5 to 3.75 V and reducing the voltage similarly in increments until the voltage reached 3.75 to 0.5 V. The measured sample is the nonporous carbon B which was prepared using petroleum-based needle coke as raw material carbon and thermally processing the carbon in a reducing ambient. Symbols, such as “+504H” attached to each sample which have undergone measurements, have the aforementioned meanings. It can be seen from the graph that each sample showed a higher capacitance when the applied voltage was reduced than when the voltage was increased by a hysteresis effect. It is observed that a sample thermally treated at a higher temperature shows a higher rising voltage and a smaller capacitance. This is due to the interplanar spacing d002. This gives the designer less latitude in designing the electrical double-layer capacitor. For example, a limitation is placed on the selection of a solvent for an electrolyte solution having a small molar volume.
Where the maximum voltage applied between the electrodes is 3.75 to 4.0 V, if a higher voltage is applied, the internal resistance will increase rapidly. It is highly likely that, if the voltage is subsequently lowered, the internal resistance is not reduced. Therefore, the maximum applied voltage is urged to be set to 3.75 V or 4.0 V. The operating voltage is set to a value (e.g., 2.7 V or 3.5 V) lower than the maximum voltage. As can be seen from FIG. 2, those materials which were processed at lower temperatures, have greater interplanar spacing d002, and lower intercalation start voltages, show greater hysteresis effects than those processed at higher temperatures and exhibit greater capacitances at low voltages.
Therefore, it is required that residual functional groups be removed as much as possible at the lowest temperature achievable.
Where a carbon material, such as activated carbon, porous carbon, or nonporous carbon, is thermally treated industrially, if the mixture gas of oxygen within the air and leaking hydrogen gas (or if oxygen leaks into a hydrogen stream from outside) is heated above 550° C., or if the mixture gas catches fire, a so-called explosive reaction of oxygen and hydrogen gases will be induced. Therefore, care must be exercised. Where the process can be performed at a temperature sufficiently lower than 550° C., the safety is enhanced, and no extra equipment cost is required.